🧹 Add missing header and cleanup old code

[burgholzer]

Description

This tiny PR adds a header that was missing for compiling with gcc and removes some special handling from the times where Barrier Statements were still considered non-unitary.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (10dec73) 92.7% compared to head (fbfd5ab) 92.8%.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #343     +/-   ##
=======================================
+ Coverage   92.7%   92.8%   +0.1%     
=======================================
  Files         35      35             
  Lines       2666    2664      -2     
  Branches     346     345      -1     
=======================================
+ Hits        2473    2474      +1     
+ Misses       193     190      -3     

Flag	Coverage Δ
cpp	94.7% <100.0%> (+0.1%)	⬆️
python	87.7% <ø> (ø)

Files	Coverage Δ
src/CircuitSimulator.cpp	92.7% <100.0%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes